Liquid detergent products present a challenge to formulators when it comes to structuring the compositions. One particular purpose of providing distinctive structure is to provide specific flow behavior. Specific types of applications often require specific flow behavior. Another common purpose of providing structure is to enable suspending solid particles in the detergent matrix, or dispersing liquids which are immiscible in the detergent matrix. In non-structured liquid detergent or personal care products, the presence of such ingredients generally leads to sedimentation or phase separation and therefore renders such detergents unacceptable from a consumer's viewpoint.
Hence, two structuring properties are typically desired in liquid detergent and personal care products: shear thinning capabilities and bead and/or particle suspension capabilities. The capability to suspend particles in principle is characterized by the yield stress value. High zero-shear viscosity values may also be indicative of particle suspension capability. Shear thinning capabilities are typically characterized by the pouring viscosity and the ratio of the pouring viscosity and low-stress viscosity values. As will be understood, the ability of a certain structuring agent to provide shear thinning capabilities alone is insufficient to determine whether the liquid product is capable of suspending bead particles with sufficient stability and vice versa.
Structuring benefits are desired at as low a level of external structurant as possible for cost and formulation concerns. For example, excessive amounts of external structuring agent may provide the particle suspension capability but result in the liquid composition becoming overly viscous and non-pourable.
It is also relevant that a structuring agent can be applied in highly concentrated liquid detergent compositions, which have low dosage volumes with high cleaning performance. Many attempts have been and still are made to produce concentrated products containing less than 50% water and high active ingredient levels. These low dosage concentrated products are in high demand since they conserve resources and can be sold in small packages. The stabilization of liquid detergent products containing very high levels of surfactants and other active ingredients and lower levels of water has proven to be particularly challenging.
A further relevant trend seen in the field of liquid detergent products is the increasing demand for bio-based products, to reduce the environmental impact of the products.
Conventional approaches for providing distinctive structure to liquid detergent and personal care products include the addition of specific structuring agents, including both internal and external structuring agents. Examples of known internal structuring agents include: surfactants and electrolytes. External structuring agents include polymers or gums, many of which are known to swell or expand when hydrated to form random dispersion of independent microgel particles. Examples include acrylate polymers, structuring gums (e.g., xanthan gum), starch, agar, hydroxyl alkyl cellulose etc. Although gums have been used to provide structuring benefits, the gums are pH dependant, i.e. failing at pH above 10. The stability of gums is also unsatisfactory at high electrolyte concentrations. Further, certain gums have been found to be susceptible to degradation in the presence of detersive enzymes. Thus, there remains a need for other external structuring agents less susceptible to these and other known problems. When large particles are suspended (e.g., polyethylene particles, guar beads), levels of polymer used is typically 1% or more.
It has previously been shown that when certain fibrous polymers (e.g., micro fibrous cellulose with large aspect ratios) are used as structurants, these may provide efficient suspending properties even at polymer levels as low as 0.1% (see e.g. U.S. Pat. No. 7,776,807, US2008/0108541 and US2008/0146485). The fibrous polymers are believed to form spider-network like structures which efficiently trap the particles inside the network and thereby impart good suspending properties. The polymers are said to provide excellent rheological properties and are said to be salt tolerant if salt is used in the formulation.
Another material reported to provide structuring benefits is bacterial cellulose. Bacterial cellulose is typically cultured using a bacterial strain of Acetobacter aceti var. xylinum and dried using spray drying or freeze drying techniques. Attempts to manufacture and prepare the dried bacterial cellulose compositions which can be rehydrated and activated into a particulate cellulose material for use in end products are known.
WO2009101545 describes an external structuring agent for use in liquid detergent products that comprises a bacterial cellulose network. This external structuring agent is said to provide both shear thinning capabilities and particle suspension capabilities.
According to WO2012/065924 and WO2012/065925 external structuring agents based on micro fibrous cellulose, such as in particular bacterial cellulose, have a zero or near zero stress-shear rate profile (i.e., zero stress-shear rate slope when plotting shear rate versus stress), resulting in flow instability and shear banding. According to WO2012/065924 these flow instability problems can be reduced or eliminated by the addition of low molecular weight water soluble polymers to the compositions comprising microfibrous (bacterial) cellulose. WO2012/065925 teaches to overcome the flow instability problems by the addition of citrus fibre to the compositions comprising microfibrous (bacterial) cellulose as an external structuring agent. The citrus fibre according to WO2012/065925 is obtained by extraction of peels and vesicles in the pulp of citrus fruit that remains after removal of the sugars to leave mainly insoluble hemicellulose.
Apart from the flow instability problems bacterial cellulose also has the obvious disadvantage that it is a relatively expensive material.
WO2012/052306 concerns laundry detergent products containing enzymes with cellulase activity. WO2012/052306 teaches to employ citrus fibre as an external structurant because it can be employed at much higher levels than bacterial MFC due to its lower cost and lower efficacy as a structurant, which is said to confer the advantage of greater resistance to destabilisation under the influence of cellulase. At a level of 0.12% the citrus fibre material did not provide sufficient susepension capability. WO2012/052306 furthermore does not address the issue of flow instability and shear banding.
To date, no liquid detergent or personal care products containing any of these types of cellulose materials as external structuring agent have become available commercially. This may be cost-related and/or the consequence of certain shortcomings of these materials in practice, e.g. in relation to performance, stability, etc.
There still remains a need for an external structuring agent which provides shear thinning capabilities and sufficient stability and particle suspension capabilities in liquid detergent and personal care products while avoiding one or more of the above mentioned problems encountered with prior art formulations.